In the discussion that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
In conventional nuclear reactors, fuel loading in the fuel elements is typically uniform along a longitudinal direction of the fuel assemblies. During irradiation in the reactor, the fuel expands due to, for example, the production of fission products and, in particular, fission products in the form of gas. The expanded fuel expands within the available space of the inner diameter of a cladding of an individual fuel element. However, over time and at higher burn-up values, the expansion of the fuel can strain the cladding, particularly where gas retention occurs and when fission products (gas or solid) begin to fill voids within the fuel. At this point, cladding strain may become proportional to burn-up and cladding strain begins to increase, quickly. This strain ultimately limits the life of fuel elements in the reactor core as expansion of the fuel cladding leads to decreased (sometimes non-uniform) coolant flow areas external to the cladding. The rate of strain is increased by the constant effect of radiation on the structural material (cladding and fuel assembly ducts). The fuel elements can expand enough to impart further strain on the duct wall of their associated fuel assemblies, which may become ‘jammed’ together due to the swelling and/or cause bowing of the fuel assembly. The fuel element swelling may sometimes cause cracks in the cladding which can lead to uncontrolled release of fission products and/or coolant interaction with the fuel. At least in part due to the resulting strain, the maximum burn-up of any particular fuel element can set the useful lifetime of a fuel element and/or the entire fuel assembly.